This invention relates to a process for the preparation of azomethines from cyclohexanone and primary aromatic amines in the presence of acid ion exchangers.
The condensation of cyclohexanone and aniline to N-cyclohexylidene-aniline with zinc chloride-aniline has long been known (Berichte der Deutschen chemischen Gesellschaft, Volume 53 (1920), pages 345 to 354). A process for the preparation of imines, using aromatic amines and aliphatic, cycloaliphatic or aromatic ketones as the starting materials, in which the reaction is carried out by circulating a mixture of the two reagents through a stationary bed of molecular sieves is also known from German Published Application No. 2,244,238; aniline and cyclohexanone are mentioned, inter alia, as starting materials. However, these known processes are subject to disadvantages which are serious for carrying out the processes industrially. In the first case, expensive working up is necessary in order to isolate the reaction product; in the latter case the water absorption capacity of the molecular sieves is restricted by their nature and after a certain time expensive desorption of water and organic material must follow.
In addition, Example 6 of German Published Application No. 1,493,942 describes the preparation of a small amount of N-cyclohexylidene-aniline by heating aniline and cyclohexanone in a molar ratio of 1 : 5 at 100.degree. to 110.degree. C. for 1.5 hours, the water which is eliminated being distilled off. This process also cannot be used on an industrial scale, since, according to our own experiments, the rate of reaction decreases when there is a smaller excess of cyclohexanone and the rate of evaporation of the water formed decreases with larger reaction volumes as a result of the longer diffusion path; moreover, because of the large excess of cyclohexanone, considerable technical effort is necessary for apparatus of the appropriate dimensions and for separating off the excess cyclohexanone by distillation.